This invention relates generally to drilling and more specifically to drilling of cooling holes in turbine airfoil components with a laser and an electro discharge machine (EDM) without the use of a beam blocking material such as wax.
There are many hole making techniques that are used for making cooling holes in turbine airfoil components. These techniques include, for example, laser, EDM, rotating EDM, shaped tube electrolytic machining (STEM) and electrochemical streaming (ES) hole drilling. Laser drilling is the fastest of the hole making techniques but has the problem of potentially damaging the surface of the opposing wall in a hollow airfoil, for example. In most conventional laser drilling processes in hollow airfoils, wax is used to attenuate the laser beam before reaching the wall opposite the drilled hole, preventing damage to the opposing wall. A drawback of this method, however, is that the process of adding and removing the wax is costly in the drilling process. In addition, if laser parameters are not properly selected, the formation of recast (liquid metal that has resolidified on the wall of the hole) on the hole being drilled can lead to cracking during the life of a rotating part. Furthermore, improper laser parameters can lead to having a hole drilled with a taper.
A rotating EDM can drill a small cooling hole at approximately 1.9 inches/minute. This rate is over two orders of magnitude faster than EDM with plunge electrodes. In most super alloys, the time for laser hole drilling is over an order of magnitude faster than rotating EDM. One drawback of the plunge electrode EDM is that the drilling speed is decreased for high aspect ratio holes, for example, small diameter and large depth. This reduction of speed is due to difficulties in expelling eroded metal debris out of the drilling area.
Accordingly, there is a need in the art for an improved method of drilling cooling holes in turbine airfoil components.
An apparatus for drilling cooling holes through a wall of a hollow component is disclosed. First, a laser drill is used for drilling at least one rough blind hole within a component wall, for example a turbine airfoil, so as to retain a residual bottom plate portion. Next, an electro discharge machine (EDM), for example a rotating EDM or plunge EDM, is used for finishing at least one rough blind hole so as to create at least one finished hole and breaking through the bottom plate portion.